The teams behind the two general-purpose detectors of the Large Hadron Collider, ATLAS and CMS, tend to go through their results when facing a deadline, usually provided by one of the large physics conferences. Last week, the Moriond conference took place in Italy and, as expected, there were a number of updates on the Higgs based on the full data collected over the past year. So far, Peter Higgs and the others that first added the Higgs mechanism to physics' Standard Model are looking pretty good.

When the LHC teams announced that they'd found a new particle this summer, they were very careful to note a caveat. While it showed up at an energy where you might expect to see a Higgs (125GeV or so), they weren't certain it was a Higgs. With the additional data, the uncertainty is beginning to fall by the wayside. As a variety of releases (here's a couple of examples) this week have indicated, the Higgs particle was predicted to have a spin of 0 and be even parity. Further studies of the 125GeV particle produced in the LHC have indicated that it has both of these.

Meanwhile, the numbers of Higgs like particles detected by different decay pathways (two photons, four leptons, etc.) are all within the range of predictions. The ATLAS detector has posted a number of animations that show how the signal at 125GeV appeared in various decay channels as the data piled up, rising above the background of other Standard Model events. This one, which shows the Higgs-ZZ decay pathway, is especially clear. There were also some hints that the Higgs was decaying into two photons more often than expected, but physicist Matt Strassler notes the additional data has pretty much eliminated that prospect.

Combined, everything seems to be pointing at a Higgs that's just what the Standard Model ordered. That doesn't mean that there aren't any other Higgs-like particles out there waiting to discover, just that we don't need them to get the sort of behavior predicted by the Standard Model. And the Standard Model still doesn't explain dark matter of the preponderance of regular matter over antimatter, so there's still the chance that the LHC has other surprises in store for us when it starts up again at a higher energy.

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"Confirmed! Newfound Particle Is the Higgs" says Yahoo. Thank you Ars, for keeping things in perspective. Apparently some of these other news sites havent learned from the mistake of "confirming" that a few neutrinos were traveling faster than the speed of light.

There was an interesting article in the New Scientist. Special and General relativity are built on the principle that intertial mass (mass due to a moving object) and gravitational mass are equivalent -- hence the rocket thought experiment. The New Scientist article suggested that this may not be the case, but that they are very close (just like Gallilean Mechanics vs Special Relativity, where for speeds much lower than the speed of light the two are nearly identical). Thus, the existence of dark matter may not be needed to explain the large scale universe structure -- it could be the increased discrepency between Einsteinian physics and a better model that takes into account the difference between inertial and gravitational mass (if the hypothesis pans out), just like Einstein was able to predict the orbit of Mercury and light curving around a planet.

Another area that could shed light into a better physical model is in black holes. Currently Einstein's equations break down at the center of a black hole, so clearly some quantum mechanical effect needs to take place (maybe, if length is quantised, it cannot be 0 and therefore Einstein's equations don't break down because you can never get to the point where it evaluates mass/0). Also, with Hawking Radiation (a black hole radiates heat at the event horizon) there is also a thermodynamic effect taking place.

"Interactions with the Higgs field provide everything but photons and gluons with mass."

Neutrinos, while massive, do not get their masses from the Higgs (or if they do, the Standard Model is dead).

Protons and neutrons also only get a very small fraction of their mass from the Higgs field. Most of their mass comes from the strong force binding energy that holds their constituent quarks (and quark-antiquark pairs, and gluons) together.

"Interactions with the Higgs field provide everything but photons and gluons with mass."

Neutrinos, while massive, do not get their masses from the Higgs (or if they do, the Standard Model is dead).

Protons and neutrons also only get a very small fraction of their mass from the Higgs field. Most of their mass comes from the strong force binding energy that holds their constituent quarks (and quark-antiquark pairs, and gluons) together.

True when talking about relativistic mass (what everyone is thinking of when referring to the 'mass of a proton'), but in this context we're talking about the intrinsic mass of fundamental particles -- the stuff in that Standard Model chart, not the stuff made from it like protons.

There is apparently an issue with the Higgs mechanism providing the rest mass of neutrinos and getting it to match the extremely tiny mass of the neutrino and relative interaction strengths of other particles.

There is apparently an issue with the Higgs mechanism providing the rest mass of neutrinos and getting it to match the extremely tiny mass of the neutrino and relative interaction strengths of other particles.

It's an issue that can be viewed as coming from the relative charges of quarks and leptons. The left handed charged leptons and neutrinos sit together in 2-component objects (we know this must be true from observations of beta decay), and at the end of the day, in order to get the down quark to have charge -1/3, the up quark +2/3, the electron -1, and the neutrino no charge at all, the neutrino must have exactly 0 interaction with the Higgs as a byproduct. Moreover, the neutrino can't pick up a mass from the Higgs without having a right handed component, which it does not in the Standard Model (and this right handed component is at the very least sufficiently different that it has never been detected and may not exist). A neutrino-Higgs interaction is unworkable without a complete overhaul of the SM.

I have an honest question. Why do physicist insist calling photons, or for the same matter, neutrinos massless? I know the photon mass is negligible but it certainly has mass, otherwise surely solar sails won't work, or am I incorrect?

I know determining the mass accurately is impossible because of Heisenburg's uncertainty principle, but calling them massless and hence no interaction with Higgs is oversimplification no?

I have an honest question. Why do physicist insist calling photons, or for the same matter, neutrinos massless? I know the photon mass is negligible but it certainly has mass, otherwise surely solar sails won't work. I know determining the mass accurately is impossible because of Heisenburg's uncertainty principle, but calling them massless and hence no interaction with Higgs is oversimplification no?

Photons have momentum without mass. Electromagnetic waves can carry (and transfer) momentum. This momentum transfer is what propels solar sails (along with the momentum of massive particles in the solar wind).

GR tells us that they must be massless. Photons in a vacuum travel at the speed of light (by definition). In order for that something to travel at the speed of light, it cannot have mass, or else it would require infinite energy to accelerate it to the speed of light.

As for neutrinos, we thought they too were massless, until we discovered their oscillations. For something to oscillate, it must experience time. If time is passing in the neutrino's reference frame, it cannot be traveling at exactly the speed of light. If it isn't travelling at the speed of light, it therefore isn't massless. Addendum: A lot of current work in physics revolves around weighing the neutrino, or rather placing an upper bound on its mass. We know it's somewhere between zero and a quarter of a millionth of the rest mass of an electron.

I have an honest question. Why do physicist insist calling photons, or for the same matter, neutrinos massless? I know the photon mass is negligible but it certainly has mass, otherwise surely solar sails won't work, or am I incorrect?

The photon mass is limited by experiment to something like 10^-22 electron masses as an upper bound. It could have a mass, but is absolutely forbidden to have one in the Standard Model. If the photon is actually very slightly massive, it must pick up a tiny mass from some extremely high energy phenomenon that we do not understand. If this is so, the speed of light is not the same as the universal speed limit c, but the properties of the photon would not be very different from those of a massless particle except on hyper-extreme distance scales (like, larger than the observable Universe).

The neutrinos are also massless in the SM, but are experimentally determined to have masses around 10^-7 to 10^-5 electron masses. There are numerous hypotheses for how they pick up their masses, which are currently being probed by experiments.

A solar sail requires photons to have *momentum*, not mass. The Newtonian approximation of momentum p=mv is an approximation that works for massive particles at low velocities. Photons are neither massive nor do they travel at low velocity. The actual formula for momentum is:

p = sqrt{(E/c)^2-(mc)^2}

for a particle with energy E and mass m. The photon has mass 0 (or so close to 0 that it might as well be 0) so that the formula for its momentum is:

p = E/c

so a photon of energy E has momentum E/c. Thus when a photon bangs into a solar sail, it transfers momentum to the spacecraft, causing it to move.

Important Pedantic Correction: it's m0 in this equation. The mass here is the intrinsic mass.

I'm applying this pedantic correction because there's already been confusion in this thread about relativistic vs intrinsic mass, and that confusion has caused some people in the past to think that E=mc2 is the "incomplete" form of the same equation, when it is not because m != m0.

Photons have no intrinsic mass, but if you had a photon gas inside a sealed container then the (relativistic) mass of that container as measured by a scale would be greater by an amount directly proportional to the energy in those photons.

Anyway, hopefully context is now firmly established and we can continue talking about intrinsic mass by just saying "mass".

Important Pedantic Correction: it's m0 in this equation. The mass here is the intrinsic mass.

The idea of relativistic mass is outdated (by about 3 decades), because it caused confusion and because the quantity of most physical interest is what you call m_0 since m_0^2 is the dot product of the energy momentum four vector. Nowadays mass always means m_0, and saying "relativistic mass" is nonsense. Your photon gas would be assigned an m_0 that is the mass *of the system*.

The idea of relativistic mass is outdated (by about 3 decades), because it caused confusion and because the quantity of most physical interest is what you call m_0

... unless you're talking about literally anything except particle physics. In astronomy, the mass of a heavenly body is not the intrinsic mass -- we can't measure that directly, and in the case of black holes it's not just unknown but unknowable. When you weigh something on a scale and say it has a mass of x kg, that mass is not the intrinsic mass. The mass of an engine that a rocket scientists cares about is not the intrinsic mass of the particles therein. Even physics experiments involving particles -- the measurements of a proton's mass via its deflection in a magnetic field was not the intrinsic mass of its constituent quarks.

Not talking about relativistic mass causes confusion because every single notion that anyone has about mass, and every calculation or measurement of mass done by someone not dealing specifically with fundamental particles, is the relativistic mass. And when you say "mass" without qualification or explanation, they think you're talking about what they think of as mass when you're not.

So it may not be interesting or useful to talk about in a physics graduate program, but it is a useful concept to discuss in general. Once people have internalized E=mc2, you can just talk exclusively about energy when relativistic mass would be applied (though this would still just be in particle physics).

Quote:

saying "relativistic mass" is nonsense.

No, it's not nonsense. It's a meaningful and correct term. Whether or not it's useful to talk about is a separate issue.

Quote:

http://en.wikipedia.org/wiki/Relativistic_mass#Controversy

An alternative point of view (in order to demonstrate that it is, in fact, a controversy):

When you weigh something on a scale and say it has a mass of x kg, that mass is not the intrinsic mass.

Yes it is. Sum all the energy momentum four vectors in a system and take the square root of the dot product. That's what you get when using a scale, and that's the "intrinsic mass" of the system. What you don't get is the sum of the individual particles' "intrinsic masses", which is why I'm calling it the system's mass.

I have an honest question. Why do physicist insist calling photons, or for the same matter, neutrinos massless? I know the photon mass is negligible but it certainly has mass, otherwise surely solar sails won't work. I know determining the mass accurately is impossible because of Heisenburg's uncertainty principle, but calling them massless and hence no interaction with Higgs is oversimplification no?

Photons have momentum without mass. Electromagnetic waves can carry (and transfer) momentum. This momentum transfer is what propels solar sails (along with the momentum of massive particles in the solar wind).

GR tells us that they must be massless. Photons in a vacuum travel at the speed of light (by definition). In order for that something to travel at the speed of light, it cannot have mass, or else it would require infinite energy to accelerate it to the speed of light.

As for neutrinos, we thought they too were massless, until we discovered their oscillations. For something to oscillate, it must experience time. If time is passing in the neutrino's reference frame, it cannot be traveling at exactly the speed of light. If it isn't travelling at the speed of light, it therefore isn't massless. Addendum: A lot of current work in physics revolves around weighing the neutrino, or rather placing an upper bound on its mass. We know it's somewhere between zero and a quarter of a millionth of the rest mass of an electron.

Voted up but just wanted to add a note of thanks for the informative post as well! I do enjoy the comments sections for Ars particle physics articles - there's usually a good few that bridge the gap between 'simplified verging on content free' and 'omg maths my brain hurts'

Back to "desperately seeking Susy". Which, amusingly, was an expression used by a speaker summing up the Moriond conference.

I want to see Susy doing splits. Unless you hark for "naturalness" I gather "split Susy" is one of the natural Susy models out there as it is getting the Higgs mass et cetera without breaking sweat. The Moriond speakers discussed some others (or maybe the same, because you need to be an expert to remember all the constructions).

Also, the accelerator program seems straight forward now, augmenting LHC over the years while Japan gets some of the electron colliders. If they get the money, that is.

I know I will come over as critical, but NS is now known for hardly _ever_ printing an interesting article since the early 00's.

A sensationalist, unverified to bordering on woo article, sure. But interesting as regards actual science, seldom.

msclrhd wrote:

Thus, the existence of dark matter may not be needed to explain the large scale universe structure -- it could be the increased discrepency between Einsteinian physics and a better model that takes into account the difference between inertial and gravitational mass (if the hypothesis pans out), just like Einstein was able to predict the orbit of Mercury and light curving around a planet.

All "modified gravity" theories predicting dark matter are known to fail, which the term "matter" suggests the basic difficulty of. You can fit such a theory to one observation, but then it will fail elsewhere. Only matter clumps like that.

msclrhd wrote:

if length is quantised

I find it doubtful since every instance of using cosmological photon's timing or polarization differences from supernova events et cetera ends up, sometimes with over 3 sigma, with spacetime being smooth below Planck scales. Our first Planck scale probes, by the way, so they will have some authority if accepted.

Just because the Planck constant decides the momentum scales of particle fields and so decides when the vacuum breaks down, it isn't tied to spacetime in any fundamental way that I know of. They could be different phenomena for all I know.*

[* And isn't that what string theory says too, the worldsheet that a string spans (disregarding branes for the moment) is not the same as "the string", you have both?]

Thus, the existence of dark matter may not be needed to explain the large scale universe structure -- it could be the increased discrepency between Einsteinian physics and a better model that takes into account the difference between inertial and gravitational mass (if the hypothesis pans out), just like Einstein was able to predict the orbit of Mercury and light curving around a planet.

All "modified gravity" theories predicting dark matter are known to fail, which the term "matter" suggests the basic difficulty of. You can fit such a theory to one observation, but then it will fail elsewhere. Only matter clumps like that.

I didn't say that this "modified gravity" _hypothesis_ [*] would predict dark matter, just the opposite.

Torbjörn Larsson, OM wrote:

msclrhd wrote:

if length is quantised

I find it doubtful since every instance of using cosmological photon's timing or polarization differences from supernova events et cetera ends up, sometimes with over 3 sigma, with spacetime being smooth below Planck scales. Our first Planck scale probes, by the way, so they will have some authority if accepted.

I wasn't aware of this. Thanks for the info.

[*] It is a hypothesis until there is experimental/test/observed data to back it up (like GR and QM have), just like string theory is not actually a theory but a hypothesis (nothing it adds to the standard model have been proven).

Yes it is. Sum all the energy momentum four vectors in a system and take the square root of the dot product. That's what you get when using a scale, and that's the "intrinsic mass" of the system. What you don't get is the sum of the individual particles' "intrinsic masses", which is why I'm calling it the system's mass.

Well screw me blue with a pool cue. So the intrinsic mass is the same as the relativistic mass in a frame co-moving with the center of momentum, and the intrinsic mass of component particles is just another source of energy for E=mc^2. Well that makes a lot of sense. Thanks for the learning.